It is well known in the art of mounting components on printed circuit boards that surface mounting technology (SMT) provides advantages in quickness of installation and practicality over the prior custom of mounting electrical components through holes on circuit boards. SMT eliminated the need for holes and, in many cases, the need for conductor leads, greatly diminishing the area required for each component. SMT thereby made possible further miniaturization. Bonding materials such as solder paste or conductive epoxy were usually applied to the contact areas on the circuit board before the components were placed thereon.
The application of the bonding material typically involves stamping, dispensing, or screening the bonding material onto the mounting surface. The stamping method requires the use of a pin, the end of which is dipped into the solder paste and then used to "stamp" the paste onto the contacts of the printed circuit board. The application is analogous to the common rubber date stamp used to print the date on a sheet of paper. The amount of paste is controlled by the size of the pin end and the thickness of the layer of paste on the "pad". The dispensing method, on the other hand, requires the use of a container having a piston and hypodermic tubing for squeezing solder paste or conductive epoxy onto the intended contact area. The amount of the paste is controlled by the diameter of the tubing, the pressure on the piston, and the duration of the squeezing. Finally, the screening technique requires the use of a fine mesh metal screen in which only target areas are permeable to the solder paste. The screen is positioned over the surface of the printed circuit board or substrate, and the solder paste is deposited onto the board through the permeable areas of the screen by pushing the paste across the screen with a squeegee. The screening method is by far the fastest and most precise way of applying solder paste or conductive epoxy to a printed circuit board or substrate for the extremely small contact areas of miniaturized electrical components, e.g., dual-in-line devices. The stamping or dispensing methods are not sufficiently precise for such applications because they tend to lead to "shorts" and "opens". In addition, these methods involve a slow and arduous process, especially in cases of components having numerous leads and contacts. The disadvantage of the screening method, however, is that it is primarily limited to applications involving flat surfaces such as printed circuit boards or substrates which are free from obstructions such as vertical pins, leads, or mounted components.
U.S. Pat. No. 4,784,310 provides a method for screen printing which works when another device is premounted on the circuit board. However, the screen must be precut to a sufficient size in a precise location relative to the area to be screen-etched. In other words, effort and planning are required to provide an aperture in the screen sufficient to avoid obstruction by the premounted device. A solder-impervious cap of a suitable size and height is then placed over the premounted device to protect it from damage or dislocation during the screening process. Thereafter, the application of solder paste is achieved with the use of a squeegee, which must ride over the protective cap. By this method, the patent avoids the need to reflux the pattern printed on the circuit board. However, if a further component is to be attached to the circuit board, the successive planning and preparation steps of the process must be repeated, since the additional mounted component will pose further obstruction problems and will require protection by a cap of sufficient size and height. If the board is populated with more than one component, substantial planning and effort are required to prepare the appropriate apertures and protective caps.
A need therefore exists for a fast, precise, and practical way to screen solder paste or conductive epoxy onto a circuit board which is populated with electrical components and which otherwise poses numerous difficulties of obstruction when the screening method is employed.